The present invention relates to hydrogen storage tanks and methods. More particularly, this invention relates to hydrogen storage tank adapted to contain solid-state hydrogen storage media, such as nano-porous silicon (npSi), and to methods for determining fill levels in such tanks.
Hydrogen-based fuel cell technologies are being considered for a wide variety of power applications, including but not limited to mobile applications such as vehicles as an attractive alternative to the use of petroleum-based products. Hydrogen-based fuel cells are also readily adaptable for use as energy sources in numerous and such diverse applications as cellular phones to space ships. They have the further desirable attribute of producing water vapor as their only byproduct and are thus environmentally benign. However, hydrogen storage remains a challenge because of its very low heat value per volume compared to fossil fuels. As such, efficient storage of hydrogen is vitally important for cost-effective system implementation. When compared to storage for conventional chemical fuels or electric energy sources, existing hydrogen storage technologies lack the convenience of gasoline for delivery and storage capacity (energy density per unit weight), and lack the flexibility of electrical energy stored in batteries and capacitors. Therefore, for fuel cells to reach their full commercial potential, improved hydrogen storage technologies are needed.
Prior methods of storing hydrogen fall broadly into two categories. The first category involves storing hydrogen chemically within a convenient chemical molecule, usually an aliphatic organic compound such as methane, octane, etc., and then pre-processing the fuel as needed, such as by catalytic reforming, to release elemental hydrogen plus carbon oxides. This method suffers two important drawbacks: carbon dioxide byproduct is a “greenhouse gas” that some believe contributes to global warming and is therefore environmentally undesirable; and the additional weight of the chemical molecule and the reformer reduce the efficiency of the entire process, making it less attractive from a cost and performance standpoint.
The second category involves mechanical or adsorptive storage of elemental hydrogen in one of three forms: compressed gas, cryogenically-refrigerated liquid, or chemisorbed onto active surfaces. Of these methods, compressed gas storage is the most straightforward and is a mature technology. However, compressed gas cylinders are quite heavy, needing sufficient strength to withstand pressures of many thousands of pounds per square inch. This weight is a considerable drawback for portable applications, and in any usage compressed gas cylinders must be treated with care because they represent a safety hazard.
Cryogenic storage of hydrogen is also well known, being used in industrial plants and as a rocket fuel. Liquid hydrogen is remarkably dense from a specific energy point of view (kilowatts per kilogram), but requires a considerable amount of additional energy to maintain the nearly absolute zero temperatures needed to keep hydrogen in a liquid state. Liquid hydrogen also requires a heavy mass of insulation, and these factors conspire to make cryogenic storage impractical for portable and small-scale applications.
Chemisorption as used herein means the adsorption of a given molecule onto an active surface, typically of a solid or a solid matrix. Chemisorption is typically reversible, although the energy of adsorption and the energy of desorption are usually different. Various catalysts and surface preparations are possible, providing a wide range of possible chemistries and surface properties for a given storage problem. Chemisorption of hydrogen has been studied extensively, and substances such as metal hydrides, palladium, and carbon nanotubes or activated carbon have been used to adsorb and desorb hydrogen. In particular, hydrogen storage in absorbed solids such as metal hydrides, metal oxides and other inorganic surfaces, carbon nanotubes/fibers, carbon fullerene, etc., has recently been considered as a promising method with the advantages of high volumetric hydrogen density and improved safety. However, prior hydrogen chemisorption techniques with these solid-state storage media have fallen short of the goals of efficiency, convenience, and low system cost for several reasons.
In the case of metal hydrides, metal oxides, and other inorganic surfaces, storage efficiencies typically are relatively low and the adsorption/desorption process is highly dependent upon exacting chemistry. These factors combine to make such approaches less than sufficiently robust for many commercial applications. Even so, metal hydrides are considered leading candidates for hydrogen storage. However, metal hydride materials expand during hydriding cycles to the extent that large stresses are generated on the material (typically particles) and the hydriding container, such that stringent requirements are imposed in container design. Furthermore, these stresses tend to fragment the metal hydride particles into finer and finer powders in an uncontrollable manner, resulting in material movement and segregation in the storage container, an increased tendency for entrainment of hydride fines in the hydrogen gas streams released from the particles, and the risk of plugged gas filters and high internal pressures within the hydriding container. Secondly, hydrogen charging (adsorption) and release (desorption) require heating of the metal hydride particles at a level of about 14.6 MJ/kg, which poses a significant thermal management challenge and leads to slow refueling.
In other materials, such as carbon nanotubes, the efficiency of hydrogen adsorbed per unit weight of matrix is higher than metal hydrides, metal oxides, and other inorganic surfaces, but desorption requires high temperatures that raise the risk of combustion. Additionally, the present cost of carbon nanostructures is relatively high, and control over material properties can be quite difficult in high-volume manufacturing.
Hydrogenated surfaces in silicon have also been employed, as disclosed in US. Pat. Nos. 5,604,162, 5,605,171, and 5,765,680, the disclosures of which are incorporated herein by reference. In each of these references, the adsorbed molecule is the radioactive hydrogen isotope tritium (3H), and the objective is the storage of this isotope to enable its safe transport, typically to a waste handling or storage facility, or to serve as a means for providing radioactive energy to power a light source. These prior methods of chemisorption do not, however, provide for desorption of hydrogen from a silicon storage medium. In fact, conventional methods of chemisorption are generally designed to prevent desorption. Further, these conventional methods of chemisorption fail to teach methods by which the storage capacity of a silicon matrix can be increased.
As a solution to the forgoing, a system for storage and retrieval of elemental hydrogen on a porous silicon media is described in U.S. Published Patent Application No. 2004/0241507 to Schubert et al., the disclosure of which is incorporated herein by reference. As a solid-state hydrogen storage media, nano-porous silicon (npSi) has a theoretical hydrogen capacity of about 6.6 weight percent, and has unique characteristics capable of enhancing hydrogen charging and recharging processes. As such, there is a need for hydrogen storage tanks adapted for containing npSi, as well as methods for their use and operation.